Allergic cough. Medicine to treat him.

Hi doctors, I am asking help for my 3 year old son. His weight is 14 kg . Birth weight was 2.9 kg. He usually get cold and cough and viral fever once in 2 months. But nose irritation and cough almost he suffers most. Last month he got viral fever and then cough started . It's sometime become dry sometime become wet. He mostly cough after waking up wheather in morning and in afternoon. I am damn sure he has allergy specially dust . And coughing feels lot mucus but in sometime it looks dry. He plays in dust mostly . His immune is weak so got trapped in cough cold. His nose always become stiffy itchi. Eyes itching. And his growth also suffer. Which syrup to give now. I am giving nothing now. Please tell antiallergic medicine to treat him.